When they had finished everything required by the law of the Lord, they returned to Galilee, to their own town of Nazareth. The child grew and became strong, filled with wisdom; and the favor of God was upon him.
So they went home.
After all the fuss at the temple, after the drama of that night when the baby was born, after all the drama with angels and unexpected pregnancies, they went home. Back to Nazareth.
They went home and the boy...grew up. Aside from what happens in 2:41-51, when the child was twelve years old, that's all we get. No miracles, no drama, no...well, nothing. They went home and the child grew up. Grew up well, to hear Luke describe it, but grew up.
With the dramatic flair Luke showed throughout the first chapter and most of the second, you'd think that if Luke had found any good juicy stories (see 1:3 for Luke's frank admission about doing research) about Jesus's childhood (nothing embarrassing, mind you, but a good healing or something), he'd have found a way to work it in. He did include the temple story, after all, or both of them (Simeon and Anna, and the twelve-year-old at the temple).
But, in the end, Luke doesn't give us anything else. Just...he grew up.
As has been observed, other writers were happy to fill in the gap. For example, some time in the second century, an unknown author filled the gap with a colorful piece of writing known now as the Infancy Gospel of Thomas. Jesus isn't a very nice kid, if you buy these tales. Apparently people wanted stories of Jesus's childhood, though. Why this kind of account was appealing is beyond me, I have to admit.
Aside from Luke, the gospels have Matthew's more spare account, and John's prologue (more on than another day). Mark has nothing to say on the subject at all. And nobody else, certainly not Paul, sees fit to address Jesus's childhood, or any of the other big biographical details, really. Paul has a few things to say about the meaning of the Incarnation, but doesn't seem to care much about how it happened (again, more later).
People get greedy. For all his sense of drama, Luke isn't going to feed any kind of frenzy for juicy stories that don't add up to good news. So, the nativity story ends with...he grew up.
And that's enough.